Field of the Invention
The present invention relates to an image capturing apparatus and a control method therefor, and in particular to an image capturing apparatus that assists a photographer during performing a follow shot and a control method therefor.
Description of the Related Art
There is a conventional shooting technique called a follow shot that highlights the speed of a moving subject. In the follow shot, a photographer pans a camera in accordance with the movement of a subject to obtain a still image in which the moving subject appears still while the background is streaked. In a typical follow shot, a target subject is shot by setting a longer exposure period than normal in accordance with the speed of the movement of the target subject. Japanese Patent Laid-Open No. 2010-245774 describes a configuration for a follow shot in which shooting is performed in an exposure period that secures a sufficient amount of streaking in the background.
In the follow shot, which requires a photographer to perform panning in accordance with the movement of a subject, the subject in a shot image becomes blurry if there is any difference between the speed of the movement of the subject and the panning speed as a result of performing the follow shot too fast or too slow. In view of this problem, a technique to assist the follow shot has been suggested, in particular a method of reducing the difference between the speed of the movement of a subject and the panning speed by moving a shift lens. Japanese Patent Laid-Open No. 2006-317848 discloses the following configuration. First, vibration is detected using a gyroscope, a subject is detected from motion vectors of an image, and a correction amount for positioning the detected subject at the center of the image is calculated. Then, the follow shot is performed while correcting the blur by moving an optical axis shift lens based on the calculated correction amount.
However, as the optical axis shift lens described in Japanese Patent Laid-Open No. 2006-317848 has a movable range that is predetermined, the maximum amount of correction that can be made by a blur correction system (hereinafter, “maximum correctable angle”) is limited, thereby giving rise to the possibility that correction cannot be made thoroughly under some circumstances.
Now, assume a situation shown in FIG. 9. FIG. 9 shows the speed of a subject and the panning speed of a camera as angular velocities, with the principal point serving as the center thereof. As shown in FIG. 9, in a case where an angular velocity of the subject is 30 deg/sec and a panning angular velocity of the camera is 24 deg/sec, a system needs to compensate for the difference therebetween, i.e., 6 deg/sec. The product of such an angular velocity that the system compensates for and an exposure period is a final correction angle that needs to be compensated for by the system.
Assume, for example, the case of shooting in which the maximum correctable angle for a shift lens is 0.4 degrees and an exposure period is 1/16 seconds; in this case, the final correction angle required is given by the following Expression (1).6×( 1/16)=0.375(≦0.4)  (1)In this case, correction can be made as the correction angle is equal to or smaller than the maximum correctable angle of 0.4 degrees.
Now, assume the case of shooting with an exposure period of ⅛ seconds; in this case, the final correction angle required is as follows.6×(⅛)=0.75(>0.4)  (2)In this case, correction cannot be made as the correction angle exceeds the maximum correctable angle of 0.4 degrees.
As described above, in order to obtain an image without a blurry subject, a photographer needs to adjust the panning speed in accordance with the movement of the subject also in an exposure duration.
While image capturing apparatuses equipped with an electronic viewfinder are widely used, generally it is often the case that, during the exposure, a display screen of the electronic viewfinder displays nothing, or keeps displaying the last image obtained before the start of the exposure. Under such circumstances, it is extremely difficult for the photographer to keep adjusting the panning speed in accordance with the movement of a subject. Furthermore, if nothing is displayed on the display screen of the electronic viewfinder during the exposure, a photographer who is not used to the follow shot may stop panning.